Circuit board manufacturing operations typically involve a stencil printer used to print solder paste onto a circuit board. Typically, a circuit board having a pattern of pads or other conductive surfaces onto which solder paste will be deposited is delivered into the stencil printer and one or more small holes or marks on the circuit board, called fiducials, is used to align the circuit board with a stencil or screen of the printer prior to the printing of solder paste onto the circuit board. After the circuit board is aligned, the board is raised to the stencil (or in some configurations, the stencil is lowered to the circuit board), solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures formed in the stencil and onto the board.
In some prior art stencil printers, a dispensing head delivers solder paste between first and second wiper blades, wherein during a print stroke one of the wiper blades is used to move or roll solder paste across the stencil. The first and second wiper blades are used on alternating boards to continually pass the roll of solder paste over the apertures of a stencil to print each successive circuit board. The wiper blades are typically at an angle with respect to the stencil to apply downward pressure on the solder paste to force the solder paste through the apertures of the stencil. In other prior art stencil printers, the dispensing head is pressurized to force solder paste through the apertures, and the wiper blades are employed to scrape excess solder paste from the stencil during a print stroke.
After solder paste is deposited onto the circuit board, an imaging system is employed to take images of areas of the circuit board and/or the stencil for, in certain instances, the purpose of inspecting the accuracy of the deposit of solder paste on the pads of the circuit board. Another application of the imaging system involves the aforementioned aligning of the stencil and the circuit board prior to printing in order to register the openings of the stencil with the electronic pads of the circuit board. Such imaging systems are disclosed in U.S. Pat. Nos. RE34,615 and 5,060,063, both to Freeman, which are owned by the assignee of the invention and hereby incorporated herein by reference. Some improved imaging systems are disclosed in pending application Ser. No. 11/272,192, entitled IMAGING SYSTEM AND METHOD FOR A STENCIL PRINTER, filed on Nov. 10, 2005, to Prince, which is owned by the assignee of the invention and is hereby incorporated herein by reference.
Consistent modeling of solder paste on a substrate, e.g., the circuit board, is required to facilitate the optimum two-dimensional imaging performance of the vision system, as well as subsequent inspections based on these images, irrespective of variations in geometry, definition, or general qualities of the deposit being imaged. Well-defined solder paste deposits have nearly vertical sides and relatively flat top surfaces that are perpendicular to the optical viewing axis (i.e., an axis generally perpendicular to a plane of the circuit board). Finely textured paste surfaces having this generally perpendicular orientation may be imaged with relative consistency using traditional illumination techniques that illuminate surfaces with on-axis white light. With on-axis illumination, the strongest components of scattered light from the top surface of the solder paste deposit are directed back along the optical viewing path and are collected by the imaging system.
In contrast, when on-axis illumination strikes a surface that is not generally perpendicular to the angle of incidence, the strongest components of scattered light from the surface are directed away, or off-axis, from the optical or on-axis viewing path and are not collected by the imaging system. Specifically, the sloped sides and irregular top surfaces of poorly shaped solder paste deposits are less efficiently illuminated and therefore more difficult to view using only on-axis illumination only.
With white light, a range of wavelengths of visible light is reflected either on-axis or off-axis from the surface (i.e., depending on the shape of the surface). Such white light is generally strongly reflected on-axis by pads having no solder paste deposits because such pads typically are clean and perpendicular to the viewing path. However, recent use of sacrificial and protective coatings to keep the pads of a substrate clean has decreased the level of on-axis reflection from pads having no solder paste deposits. This decrease in reflection makes differentiation between pads without solder paste deposits and with solder paste deposits more difficult.